I need to use templates to generate all sorts of things (java classes, xml and property files...), I have enough of
Velocity and I love
Groovy. Three good reasons to use
Groovy Templates.
Groovy Templates come with different flavours :
- SimpleTemplateEngine – for basic templates
- GStringTemplateEngine – stores the template as writable closures (useful for streaming scenarios)
- XmlTemplateEngine – works well when the template and output are valid XML
For what I need to do the
SimpleTemplateEngine is fine. I can then use simple logic in my templates using the Groovy syntax a bit like in JSPs :
($attribute)
<% groovy expression %>
<%= attribute %>
But there's something missing in Groovy Templates : an easy way to include templates within other templates such as
<%@ include file=""%> in JSPs. So I've created a new Template engine (called
IncludeTemplateEngine) based on the
SimpleTemplateEngine. I didn't change much the original code and I introduced a new tag
<@ fileName @> (fileName being the template that has to be included in the parent template.
This way, I can have a parent template that can include a child template like this :
parent.tmpl
Begening of the parent template
<@ child.tmpl @>
End of the parent template
The result is when you use the
IncludeTemplateEngine, the child gets included in the parent and the result is :
Begining of the parent template
I am the child template
End of the parent template
And to use the
IncludeTemplateEngine, just do as usual :
def f = new File('parent.tmpl')
def engine = new IncludeTemplateEngine()
println tmpl.make(binding).toString()
If you want the code of this template, it's not rocket science and uses 90% of the original Groovy template, but it can be useful :
import groovy.text.TemplateEngine;
import groovy.text.Template;
import groovy.lang.*;
import java.io.*;
import java.util.Map;
import org.codehaus.groovy.control.CompilationFailedException;
import org.codehaus.groovy.runtime.InvokerHelper;
/**
* This simple template engine uses JSP <% %> script, <%= %> and <@ @> expression syntax. It also lets you use normal groovy expressions in
* the template text much like the new JSP EL functionality. The variable 'out' is bound to the writer that the template is being written to.
*
* @author sam
* @author Christian Stein
* @author Paul King
*/
public class IncludeTemplateEngine extends TemplateEngine {
private boolean verbose;
private static int counter = 1;
private GroovyShell groovyShell;
public IncludeTemplateEngine() {
this(GroovyShell.class.getClassLoader());
}
public IncludeTemplateEngine(boolean verbose) {
this(GroovyShell.class.getClassLoader());
setVerbose(verbose);
}
public IncludeTemplateEngine(ClassLoader parentLoader) {
this(new GroovyShell(parentLoader));
}
public IncludeTemplateEngine(GroovyShell groovyShell) {
this.groovyShell = groovyShell;
}
public Template createTemplate(Reader reader) throws CompilationFailedException, IOException {
SimpleTemplate template = new SimpleTemplate();
String script = template.parse(reader, true);
if (verbose) {
System.out.println("\n-- script source --");
System.out.print(script);
System.out.println("\n-- script end --\n");
}
try {
template.script = groovyShell.parse(script, "SimpleTemplateScript" + counter++ + ".groovy");
} catch (Exception e) {
throw new GroovyRuntimeException("Failed to parse template script (your template may contain an error or be trying to use expressions not currently supported): " + e.getMessage());
}
return template;
}
public void setVerbose(boolean verbose) {
this.verbose = verbose;
}
public boolean isVerbose() {
return verbose;
}
private static class SimpleTemplate implements Template {
protected Script script;
public Writable make() {
return make(null);
}
public Writable make(final Map map) {
return new Writable() {
/**
* Write the template document with the set binding applied to the writer.
*
* @see groovy.lang.Writable#writeTo(java.io.Writer)
*/
public Writer writeTo(Writer writer) {
Binding binding;
if (map == null)
binding = new Binding();
else
binding = new Binding(map);
Script scriptObject = InvokerHelper.createScript(script.getClass(), binding);
PrintWriter pw = new PrintWriter(writer);
scriptObject.setProperty("out", pw);
scriptObject.run();
pw.flush();
return writer;
}
/**
* Convert the template and binding into a result String.
*
* @see java.lang.Object#toString()
*/
public String toString() {
StringWriter sw = new StringWriter();
writeTo(sw);
return sw.toString();
}
};
}
/**
* Parse the text document looking for <% or <%= and then call out to the appropriate handler, otherwise copy the text directly
* into the script while escaping quotes.
*
* @param reader a reader for the template text
* @return the parsed text
* @throws IOException if something goes wrong
*/
protected String parse(Reader reader, boolean rootTemplate) throws IOException {
if (!reader.markSupported()) {
reader = new BufferedReader(reader);
}
StringWriter sw = new StringWriter();
if (rootTemplate)
startScript(sw);
int c;
while ((c = reader.read()) != -1) {
if (c == '<') {
reader.mark(1);
c = reader.read();
if (c != '%') {
if (c == '@') { //64
c = reader.read();
processInclude(reader, sw);
} else {
sw.write('<');
reader.reset();
}
} else {
reader.mark(1);
c = reader.read();
if (c == '=') {
groovyExpression(reader, sw);
} else {
reader.reset();
groovySection(reader, sw);
}
}
continue; // at least '<' is consumed … read next chars.
}
if (c == '$') {
reader.mark(1);
c = reader.read();
if (c != '{') {
sw.write('$');
reader.reset();
} else {
reader.mark(1);
sw.write("${");
processGSstring(reader, sw);
}
continue; // at least '$' is consumed … read next chars.
}
if (c == '\"') {
sw.write('\\');
}
/*
* Handle raw new line characters.
*/
if (c == '\n' || c == '\r') {
if (c == '\r') { // on Windows, "\r\n" is a new line.
reader.mark(1);
c = reader.read();
if (c != '\n') {
reader.reset();
}
}
sw.write("\\n\");\nout.print(\"");
continue;
}
sw.write(c);
}
endScript(sw);
return sw.toString();
}
private void startScript(StringWriter sw) {
sw.write("/* Generated by SimpleTemplateEngine */\n");
sw.write("out.print(\"");
}
private void endScript(StringWriter sw) {
sw.write("\");\n");
}
private void processInclude(Reader reader, StringWriter sw) throws IOException {
int c;
StringWriter templateName = new StringWriter();
while ((c = reader.read()) != -1) {
if (c == '@') {
c = reader.read();
if (c == '>') break;
}
if (c != '\n' && c != '\r') {
templateName.write(c);
}
}
File f = new File(templateName.toString().trim());
SimpleTemplate template = new SimpleTemplate();
String script = template.parse(new FileReader(f), false);
sw.write(script);
sw.write("out.print(\"");
}
private void processGSstring(Reader reader, StringWriter sw) throws IOException {
int c;
while ((c = reader.read()) != -1) {
if (c != '\n' && c != '\r') {
sw.write(c);
}
if (c == '}') {
break;
}
}
}
/**
* Closes the currently open write and writes out the following text as a GString expression until it reaches an end %>.
*
* @param reader a reader for the template text
* @param sw a StringWriter to write expression content
* @throws IOException if something goes wrong
*/
private void groovyExpression(Reader reader, StringWriter sw) throws IOException {
sw.write("\");out.print(\"${");
int c;
while ((c = reader.read()) != -1) {
if (c == '%') {
c = reader.read();
if (c != '>') {
sw.write('%');
} else {
break;
}
}
if (c != '\n' && c != '\r') {
sw.write(c);
}
}
sw.write("}\");\nout.print(\"");
}
/**
* Closes the currently open write and writes the following text as normal Groovy script code until it reaches an end %>.
*
* @param reader a reader for the template text
* @param sw a StringWriter to write expression content
* @throws IOException if something goes wrong
*/
private void groovySection(Reader reader, StringWriter sw) throws IOException {
sw.write("\");");
int c;
while ((c = reader.read()) != -1) {
if (c == '%') {
c = reader.read();
if (c != '>') {
sw.write('%');
} else {
break;
}
}
/* Don't eat EOL chars in sections - as they are valid instruction separators.
* See http://jira.codehaus.org/browse/GROOVY-980
*/
// if (c != '\n' && c != '\r') {
sw.write(c);
//}
}
sw.write(";\nout.print(\"");
}
}
}
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